Acid mine drainage (AMD) results from the exposure and subsequent oxidation of sulfide minerals, primarily pyrite, during the mining of coal and various metal sulfide ores. The acidic reaction products are dissolved by infiltrating rainwater, eventually emerging in acid seeps or springs. While many factors influence the quantity and quality of water handled by a mining operation, it is not unusual for a single mine to treat one million gallons of acid water a day. The costs of such treatment range up to $500,000 per year, with the entire industry perhaps spending over $1 million per day.
Conventional AMD treatment processes basically comprise four steps: (1) neutralization, (2) aeration, (3) settling and disposal of sludge, and (4) effluent discharge. The neutralization and aeration steps in conventional treatment processes are separate discrete steps and, in any medium to large scale operation, necessitate equipment that requires an on site electrical power source and periodic maintenance. In addition, capital costs are high, particularly if the site is remote and power lines must be run to the site. Each step also requires a special construction to carry that step out, e.g., neutralization usually requires some type of erected structure for the storage and liquifaction of neutralizing material, and aeration generally requires either a surface erected structure or an excavation for an aeration basin and special supports for securing the mechanical aeration device in place. Typical capital costs for an AMD facility range between about $500,000 and $2 million. A discussion of commonly used, conventional processes can be found in a publication of the Environmental Protection Agency entitled "Design Manual: Neutralization of Acid Mine Drainage" (EPA-600/2-83-001, January 1983).
Aeration requirements vary, based upon iron concentrations and flow volume. Iron dissolved in the acidic water is often in the Fe.sup.2+ state and must be oxidized to Fe.sup.3+ before discharge so that it will hydrolyze and precipitate as Fe(OH).sub.3. Manganese, if present, must be also be oxidized and precipitated. The rate of the oxidation is a function of the dissolved oxygen and pH, and even if aerated, mine water contains only relatively small amounts of dissolved oxygen. To replenish the dissolved oxygen, settling ponds are made wide and shallow to maximize the diffusion of oxygen into the water. However, oxygen diffusion is relatively slow so that at many sites, supplementary aeration sources are necessary. For example, diffusion can be increased by increasing turbulence and is typically accomplished by incorporating a series of open-channel drops in the flow path of the water to produce such turbulence and thereby increase the dissolved oxygen concentration. As mentioned above, mechanical aerators are also used to continuously introduce bubbles of air into the water. However, this approach has a number of disadvantages including those outlined above, i.e., the requirement for a separate aeration tank, high initial capital costs, and high operating costs associated with power consumption and maintenance, especially where gypsum precipitation is a problem.
Patents of possible interest in this and related fields include U.S. Pat. Nos. 4,377,486 (Barrick et al); 4,329,224 (Kim); and 3,743,598 (Field).